Argon iOS synth by Icegear.
The iPhone seems to me too small to use as a live audio workstation. Yet, I appreciate the ingenuity.
440audio is a good start at bringing all the iOS audio apps together. At such time it makes it easier to figure out what apps are optimized for iPad, and, next moves to include app end-user commentary for the purpose of filtering the wheat from the chaff, the site will be even more useful.